Sample introduction apparatuses in the form of nebulisers for liquid samples are known. For example, pneumatic nebulisers, ultrasonic nebulisers, and thermospray nebulisers have been coupled to ICP-MS instruments. A nebuliser converts a liquid sample into a spray, or aerosol, which is directed to a plasma/excitation device, either for ionisation for mass spectrometry analysis downstream of the device, or for excitation for optical emission/absorption analysis in the device.
FIG. 1 shows schematically conventional ICP-MS source. A liquid is introduced into a spray chamber 10 using a nebuliser 12, which is typically driven by a flow of the same gas as the plasma gas (usually argon but sometimes helium). The spray chamber 10 may optionally incorporate a line-of-sight obstruction (not shown), to prevent direct delivery of droplets into a sample tube, or injector, 14. It may also optionally incorporate a drain (not shown) for removal of excess liquid and a cooling device (not shown). The sample tube 14 is disposed within an auxiliary gas tube 16, which is itself disposed inside a plasma torch 18. Such a torch is shown in U.S. Pat. No. 7,273,996. Surrounding the torch 18 is an induction coil 20 which is energised with an RF electric current, typically at 27 or 40 MHz. A plasma gas—typically argon—is supplied via a plasma gas inlet 22 into the torch 18 and is converted into a plasma at a plasma region 24 towards the end of the torch. The aerosol enters the torch 18 via the sample tube 14 and auxiliary gas tube 16 and, due to the high temperature of the plasma, is ionised at the plasma region 24. To help to introduce the nebulised sample into the centre of the plasma region, an auxiliary gas flow is provided via an auxiliary gas inlet 26 into the auxiliary gas tube 16, so that both the plasma gas and the auxiliary gas surround the sample stream concentrically. Finally, the sample ions are extracted from the plasma through a sampling aperture 28, to a mass analysing apparatus.
In ICP-OES, a similar configuration is used, except that the sampling aperture 28 is not required, since extraction to a mass spectrometer does not take place. Instead, optical emissions from the sample in the plasma region 24 are analysed with an optical spectrometer. Observations with the spectrometer may be made from the back or from the side of the plasma region.
It is known that the efficiency of sample ionisation or of sample excitation for emission/absorption is affected by the size and distribution in size of the sample droplets resulting from nebulisation. Large droplets and a wide distribution in droplet size lead to excessive liquid injection into the torch and consequentially instability of the plasma due to the varying load. Contamination of the sample and skimmer cones may also increase. Furthermore, because of the increased energy requirement for evaporating larger droplets, incomplete atomisation and ionisation of the sample may occur, resulting in molecular interferences.
A general approach for improving the stability of the plasma is to increase the size and power of the plasma generator, to cope with large sample droplets and variations in the droplet size. Another approach involves cooling the nebulisation spray chamber, to provide condensed droplets on its walls. This leads to a shift of the liquid/gaseous equilibrium in the spray chamber, resulting in smaller droplets, by the removal (evaporation) of solvent from the droplets to bring the partial pressure of the solvent back towards its required vapour pressure in the spray chamber as solvent condenses and is drained away.
A further approach involves providing a small diameter for the nebuliser needle bore, with the aim of providing smaller droplets into the spray chamber. However, since ICP samples frequently have a high salt content and comprise a certain proportion of unsolvated solid, precipitation of salts in the needle can result, eventually leading to blockage of the bore. Consequently, the bore diameter cannot be made very small and an additional, desolvation or dehumidification step may be introduced to try to reduce the nebulised droplet size.
The above techniques for nebulisation and desolvation involve costly spray chambers and spray chamber cooling, as well as the provision of an argon flow which may exceed what is actually needed for clean driving of the plasma itself. Despite the various developments discussed above, there is considerable room for improvement in the droplet formation technique.
There is a need therefore for an improved or alternative sample excitation apparatus and method for supplying and exciting a sample in a plasma generator, a flame, or another sample excitation device for subsequent elemental analysis thereof. In particular, it would be desirable to provide a sample excitation apparatus which comprises a standard ICP ionisation source. This invention aims to provide such an apparatus and method.